Brain needs the middle ear to track depth

October 7, 2005

When you jaywalk, your ability to keep track of that oncoming truck

despite your constantly changing position can be a lifesaver. But

scientists do not understand how such constant updating of depth and

distance takes place, suspecting that the brain receives information

not just from the eye but also from the motion-detecting vestibular

system in the middle ear.

In studies with monkeys reported in the October 6, 2005, issue of

Neuron, Nuo Li and Dora ki of Washington University School of

Medicine in St. Louis have demonstrated how such depth motion is

updated and strongly implicated the vestibular system in that

process.

In their experiments, the researchers trained the monkeys to perform

memory-guided eye movements. The animals were first shown a light a

fixed distance away from their head. Then the researchers flashed one

of eight other, closer " world-fixed " target lights. Next, with the

room lights turned off, the monkeys were moved either forward or

backward and the fixed-distance light flashed, signaling the monkeys

that they should look at where they remembered the world-fixed light

had flashed. Finally, the room lights and target light were turned

on, so the monkey could make any corrective eye movement to the re-

lit target. For comparison, the researchers also conducted

experiments in which the monkeys were not moved.

Such an experimental design using passive motion enabled the

researchers to study depth-tracking in the absence of any clues the

monkeys might have gleaned from their own motor movements--leaving

vestibular system as the most likely source of information.

Finally, the researchers eliminated the vestibular systems in two of

the monkeys and performed the same eye-movement experiments.

They found that the eye motion of monkeys in the first experiments

indicated that they were clearly able to update their perception of

the depth of the target, even in the absence of information from

their own motor movements. By contrast, the monkeys that lacked

vestibular systems showed compromised ability in the task.

" These results demonstrate not only that monkeys can update retinal

disparity information but also that intact vestibular motion cues are

critical in reconstructing three-dimensional visual space during

motion in depth, " concluded Li and ki.

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